Arrangement for limiting noise interference in carrier telephone systems



Nov. 22, 1949 A. L. BONNER ARRANGEMENT FOR LIMITING NOISE INTERFERENCE IN CARRIER TELEPHONE SYSTEMS Filed Nov. 4, 1947 N www .n .man

Patented Nov. 22, 1949 ARRANGEMENT FOR LIMITING NOISE IN- TERFERENCE INv CARRIER TELEPHONE SYSTEMS Arthur L. Bonner, Chatham, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 4, 1947, Serial No. 784,022

1o claims. (c1. 179-1) The invention relates to telephone systems and particularly to built-up toll telephone systems comprising voice frequency and high frequency carrier telephone links or a plurality of voice frequency telephone links in tandem, in which alternating currents of` Voice and lower frequencies are utilized for ringing and other signaling or switching purposes.

An object of the invention is to reduce noise interference in a carrier or a voice frequency link of such a built-up telephone system due to the transmitted signaling currents.

objectionable voice interference may be pro,- duced in such a system Where a local voice irequency switching olce has control of a toll connection, as is the case where decentralized switching is employed, when the local operator sends a 20-cycle ringing signal out over a trunk line to operate an alternating current control relay in the toll terminal oliice, which controls the operation of a ringer to send a disconnect or rering signal to distant toll operators over an associated carrier or other link. One source of interference is the audible ring-back tone superimposed on the 20cycle ringing signal, which is used in local connections to inform the calling subscriber that the called subscriber is being rung but has no useful function in toll connections. This tone, which is composed mostly of frequencies between 200 and 500 cycles, is sent back to the calling subscriber through his loop circuit, but as a modulation of the 20-cycle ringing signal is also impressed directly on the trunk line leading to the toll oflice where it is impressed on an associated carrier or voice frequency telephone link. Other signaling tones, such as all paths busy and zip tones are impressed on the toll lines with the level at which they are used, but the audible ring-back tone usually has a level which is 50 to 60 decibels greater than the usable level so that it is one of the worst sources of interference impressed on the carrier link.

A second factor involved in the ringing interference is the modulation of the 20-cycle signal'- ing Wave caused by saturation of the signaling control relay at the toll oice end of the trunk,

to produce harmonics of 20 cycles (mainly 100, 140 and 180 cycles) which in some cases have amplitudes nearly as large as the worst audible ringing components. A third factor is the transient pulses which occur when the 20-cycle ringing voltage is applied and 'removed from the circuit. Although these transients are of short duration, their peak voltages may be very high if the amplitude of ,the ringing voltage wave is a.

. 2 Y maximum at the time the vcircuit is made or broken.

Limitation of such interference is desirable from the standpoint of proper operation of such a system employinga carrier link. The power of the ringing signals is much greater than the total voice load and suicient to overload the associated carrier channel and cause spurious noise interference through cross modulation in other channels of the samesystem. It also greatly increases the oost of gain regulation to make the carrier system sufficiently stable to transmit voice frequency telegraph signals.

In accordance with one embodiment of the invention, the effects of such interference on the operation of a carrier link in such a built-up system are minimized by the use at the terminal toll office of a circuit arrangement operating to effectively disable the outgoing carrier transmission path of an associated carrier channel only during periods in which the ringing signals are being received over the trunk line from a preceding local Yvoice frequency switching office. One arrangement of this embodiment employs two varistor circuits and a control relay having a predetermined operating time. One varistor circuit which is connected across the input of the outgoing transmission path of the carrier link, operating as a vario-losser, normally provides a high impedance shunt thereon so that its effect on the transmission of voice frequencies during talking periods is very small. The second varistor circuit connected to the incoming toll line and.

tuned to 20 cycles, operates as a rectifier to selectivelyy rectify the Ztl-cycle components of the incoming signals during signaling periods. The rectified signals'are applied through the winding of the control relay as a bias to the varistors in the first varistor circuit in such direction as to reduce Vtheir Y impedance values to low values thereby lowering the shunting impedance of the latter circuit on` the carrier path so as to insert quickly anappreciable loss in that path to the applied signaling currents. The relay operates with a predetermined amount of delay after initial operation of the vario-losser, in response to the applied rectified signals to completely short the input of the carriertransmission path so as to prevent thereafter any appreciable amount of the impressed signaling current of 20 cycles and voice frequencies, and all higher frequency transients due to signaling, from being transmitted out over that path. v

In accordance with another embodiment of the invention,v the completeinterference limiting are rangement including the vario-losser and the rectifier control arrangement therefor is inserted in the two-wire path on the trunk line side of a hybrid coil coupling that two-wire path to the incoming and outgoing paths of the carrier channel, and operatesin` a manner similar tothat of, above-described arrangement to effectively suppress the signaling currents and harmonics thereof from the carrier channel during signal; ing intervals.

The various objects and featuresor the innention will be better understood from the following detailed description when read in -conjunction with the accompanying drawing, Fig. 1 oiv which. shows schematically a built-up telephonesystemV embodying the invention and Fig. 2 shows schematically a modied arrangement in, ace.

cordance with the invention applied to the builtup telephone system at a different pointe OnlyT sufficient parts of the built-up telephone system necessary to a proper description ofthe invention are illustrated in the drawing, shown, this system includes a telephone subscriber station I,v which, when it isy involved in a toll call, is adapted through theassociated subscribersloop circuit to be connected by the cord circuit 2 of the local voice frequency switching oince A to a trunk line 3 extending to ardistant toll terminal station B. The toll terminal station B is connected or adapted to be connected in well-known manner by a four-wire terminating set comprising a hybrid coil H. andassociated line balancing network N to theoutgoing trans-n mission circuit` TC including the channel modulating unit M and associated carrier oscillator O1, and incoming transmission circuit RC irl-,- cluding the channel demodulating unit DM and associated carrier oscillator. O2, ofi4 one two-Way carrier channel CC of a carrier link. The carrier link may include other two-Way. carrierrchannels CCI each of. which may be connectedy to other toll terminal equipments similar to that above described in similar manner. The carrier-v channels may be connected through two-.wireor four-wire paths, or a radio path, to the far end (not shown) of the carrier link, which` may be connected through similar toll. terminal oilces, trunk lines and local voice frequency switching oices to other subscribers telephone stations. to provide toll connections.

The local switching o'ice A may. include, as indicated, suitable signaling equipment including a source 4 of 2li-cycle signaling current, a source 5 of relatively high amplitude audible ring-back tone composed. mostly.Y of voice fre,- quencies inthe range from 200 to 500 cycles, and a signaling lamp 6, which may be connected in series by the local operator by. means ofthering key arrangement 7 across the cord circuiti. 2st()r send out a 20-cycleringing current, with superimposed ring-back tone, when she-wishes to-signal an associatedlocal telephone-subscriber,` or a toll subscriber over thel associated trunk linerr3.- The ring-back tone, which is made use oi only in local connections for notifyingv a calling sub- 65 scriber that the called subscriber-is being rung, is connected back to the callingwsubscribers loop circuit by the small condensers 8. Althoughthis ring-back tone has no useful functionin-a toll connection, whenthelocal. operator.. rings after- 70,

atoll connection has been set up, the high level ring-back tone will be superimposed: on.the 20 cycle ringing signal impressed. directly on the trunk 3 and transmitted thereover to` the toll 4 the hybrid coil H on the outgoing transmission path of the associated carrier channel CC.

The equipment for accomplishing the purposes of the invention located at toll terminal oice B includes the varistor bridge circuit 9, the varistor bridge Circuit t0n and arelayi IJ harina a predetermined operating timoh which` mayf be the signaling relay of the office suitably modified and with additional contacts thereon for a purpose to be described later". The varistor bridge circuit 9 is utilizedy as a full-Wave rectiiier. The particular varistor bridge circuit used for this purpose which was constructed and operated satisfactorily had equivalent dry plate rectifiers 0rvaristor-s of thecopper-oxide type, poled as indicated, in its four arms, but any other type o f full-Waverectfen-may be used. One pair of oppositeV terminals of this bride circuit, operating as the input terminals,y are connected through series condensers I2 and I3 across the condenser I4. located between. the windings of the hybrid coil. H: in series with the trunk line 3, the values of; these condensers. being selected with respect tothe inductance of the relay II so that the input circuit of the rectierY 9 is tuned to 20 cycles. The second varistor bridge circuit I0 operateseas a vario-losser. The particular bridge arrangement used for this purpose inthe constructedV circuit had equivalent dry plate rectiiiers or varistors of theA thallium copper type, poled as indicated, inY its four arms, but any other vario-losser arrangement adapted for producing the same results may be used. One pair ofv opposite bridge terminals of the bridge circuit Ii is connected through the operating winding ofthe relay II across the output terminals of the.varistorbridge circuit 9 and its other pair of opposite terminals is. connectedA across the input of the outgoing transmission circuitl TC of the carrier channel CC.

The impedances of the varistor elements in the four armsof the varistor-bridge circuit IIl are selected sothat normally, that is, when no biasing currents are supplied thereto from the output. of therrectier bridge circuit 9, the-shunting impedance. connected by the former varistor bridge. circuit across theinputof the outgoing carriertlansmssion circuit TC is high and thus the bridge normally provides a low loss in that circuit to voice currents. Since the path through theforward directionof these varistor arms includes the relayHII, thel inductance offthis relay must be high enough to provide a low bridgingloss.

The manner in which the equipment atthey tollj terminal station justI describedA functions to minimize noise interference inthe-carrier chan7 neldueto thelow frequency signaling(` currenttransmitted overt-he trunk line 3-,frorn the local voice frequency switching circuit A Will now be described;

Assume that, after the toll-connection has been setup in normal manner, the local operator at stationA desires-to sendi a disconnect or rering signal to distant toll operators. Thelocaloperatorwill operate his' ring keyarrangement 1f which connects the 20-cycle ringingsource Land; audible tonesource 5-in series-with the ringing signallamp I` across his. cordxcircuit-Z.l The-.20- cycle ringing signal `fromsource 4 with the superimposed` audibletone supplied by -f source 5f will ther'rbe transmitted` to theV cordcircuit 2. Theseries condensers 8 in the cord circuit 2l aref designed :to provide a high-10155150 to decibels)`- terminal oice B whereit will be impressedby- 75. to the\20'cycle ring-ingfrequency thuszsubstantially preventing the low level 20-cycle ringing circuit from being transmitted back into the loop circuit associated with subscriber station I, but the relative high amplitude audible ring-back tone comprising mainly the frequencies of 200 to 500 cycles will be transmitted back to. the calling subscriber through these condensers readily. The ringing signal with the superimposed audible ring-back tone'will pass from local cord circuit 2 over trunk line 3 to the terminal toll office B and a portion thereof will be impressed by the hybrid coil H on the transmitting circuit TC of the carrier channel CC. A portion of the 20- .cycle component of the incoming ringing signals will be picked olf from the hybrid coil H bythe tuned input circuit of the full-wave rectifier 9 and will be rectified by the latter. The rectified signaling currents in the output of the rectifier bridge 9 will pass through the winding ofthe slow-operate control relay l to the varistor bridge l0. The rectified currents will pass through the varistors in bridge l0 in such direction as to lower their impedances to a low value thereby providing a low impedance shunt across the input of the carrier transmitting circuit TC and thus a high loss (about 15 decibels for the particular vario-losser used) to the signaling current impressed on that circuit by hybrid coil H. A short time later (approximately .03 second with the particular relay used) the rectified signal current flowing through the winding of the slow-operate relay Il will complete operation of that relay to provide a complete short vcircuit through its operated lower switching contacts across the input of the transmitting circuit TC. This will effectively prevent the signaling currents and all higher frequency transients du`e to signaling from being thereafter transmitted out over the carrier link.

If the relay Il is also employed as the oce ringing relay, as indicated here, its operation to open its normally closed upper contacts may be utilized to cause operation of an associated ringer (not shown) to send out a disconnect or rering signal to distant toll operators over the carrier link. As stated previously the relay Il has another function which is to introduce suicient impedance to the alternating current frequency components of the rectified signal currents in the output of varistor bridge 9 so that the impedance of the vario-losser varistor bridge I0 will be kept at a high enough value so as not to provide a shunt on the carrier transmission pathduring non-signaling periods.

. A portion of the built-up telephone system of Fig. 1 including a modifiedv arrangement in accordance with the invention is shown in Fig. 2. The figure shows the two-wire path 20 con-i necting the trunk line 3 from the local voice frequency switching oiice A to the hybrid coil H coupling to the carrier channel CC. The entire interference limiting arrangement including the vario-losser bridge Ill and the rectifier bridge 9 is located in this two-wire path. The rectifier bridge 9 is connected in series with the ringing control relay I6 and the condenser I 1 in shunt of the path 29, and the vario-losser bridge l0 is connected directly in shunt to that path at a point therein nearer to the hybrid coil H. The retardation coil I is connected in series with the circuit connecting the output of the rectifier bridge 9 across the vario-losser bridge l0 to provide a high impedance to 20 cycles therebetween during signaling intervals. The necessary loss to 20 cycles between the rectier and vario-losser shunts to prevent the 20-cyc1e signaling current from being affected by the vario-losser action is obtained by tuning the elements (relay I6, condenser I1, the input circuit of rectifier 9, etc.) in series with the rectier shunt to 20 cycles, and those elements in series with the vario-losser shunt to some higher frequency. The condensers I8 and I9 in series with the path 20 between the two shunts are used to help tune the vario-losser shunt.

Various other modifications of the circuitillustrated and described which are within the spirit and scope of the invention will occur to persons skilled in the art. For example, the built-up telephone system may comprise a plurality of voice frequency links only and an arrangement such as illustrated in Fig. 1 or Fig. 2 may be employed for preventing noise interference in one of the four-wire or two-wire voice frequency links therein.

What is claimed is:

l. In combination, a telephone channel, a line for feeding voice frequency telephone signals to said channel, means for occasionally impressing auxiliary signals including voice and lower frequencies on said line for ringing or switching purposes, and means for limiting noise interference in said telephone channel due to said auxiliary signals comprising means responsive to the lower frequencies in the auxiliary signals received by said channel over said line during signaling intervals to insert a loss in the input of said channel sufficient to substantially prevent transmission thereby of said auxiliary signals and all higher frequency transients due to said auxiliary signals.

2. In combination in a telephone system, a carrier channel, a line for feeding voice frequency telephone signals to said channel for modulation therein on a carrier wave, means for occasionally impressing auxiliary signals comprising frequencies within the voice range and lower frequencies, on said line, switching means responsive to the lower frequencies in the auxiliary signals, connected to said line near the input of said carrier channel and means to limit noise interference in said carrier channel due to the transmitted auxiliary signals comprising means responsive on a frequency basis to said lower frequencies in the auxiliary signals transmitted over said line to insert a large loss in the input of said channel while the auxiliary signals are being received thereby.

3. The combination of claim 2, in which said interference limiting means includes a variolosser bridged across the input of said channel and normally providing a relatively low loss therein to voice frequency telephone signals, and

means selectively responsive to the auxiliary sig-v nals of said lower frequencies received over said line for adjusting said vario-losser so that it inserts a high loss in the input of said carrier channel during the auxiliary signal receiving intervals.

4. The combination of claim 2, in which said interference limiting means includes a variolosser bridged across the input of said carrier channel, normally providing a low loss therein to voice frequency telephone signals, means for selecting and rectifying the lower frequency components of the auxiliary signals received over said line, and for utilizing a portion of the rectified signal components to bias said vario-losser so as to increase the loss inserted thereby in the input of said carrier channel.

5. The combination of claim 2, in which said interference limiting means comprises a variodessins? losser bridged across the. inputr of. said. carrier channel andf normally 'a'dlustedtofprovideY a highi impedance'shunt. on. said channel such as; not. toappreciablyl interfere with. the: transmission of. voice frequency telephone: signals therein, means. toselect andv rectify a portionA of. thef auxiliary; signals. of said lower frequencies received over said line, means responsivezto'thezrectied signalsv for biasingsaid vario-losser so. that it providesv a.

low impedance shunt on said carrier channeliand.:

delayed. operating' auxiliary means 'also responsive to' said' rectified; signals to connect. ai directt short across said carrier vchannel so as tofeiec.- tively prevent transmission thereby ofisaid aux'- iliary signals and; all higher' frequency transients; introduced thereby entering; said; channel.

6. Thel combinationy of. claim. 2, inwhich'4 said: interference limiting-t means comprisesa vario-l l'osser bridged: across the: input'voi said channel; and normally providing a relatively low loss therein to voice frequency telephone signals,A means to select and rectify signal components of said lower frequencies from the auxiliary signals re ceivedA over said line, means-for applying, one: portion ofthe rectified signal components. as a biasi to said vario-losser to vary` its impedance so as to immediately appreciablyl increase the amount of `loss inserted thereby in said channel. and delayed operating relay means connected between the output of the-rectiying means and said variolosser responsive to thev appliedA rectified signal componentsto close aV direct short circuit across the input of said channel.,

7. The combination of claim 2, in: which saidy interference limiting means comprises one` varistor bridge circuit connected across the input. ofJsaid carrier. channel andnormallyproviding: a high impedance shuntl thereon such as not to.` appreciably aiect the transmission of` voice frequency signals, a: second varistor bridge circuit having an input circuit tuned to said lower fre-y quencies connected' to Saidline near said carrier channel, operatingas a. rectiler to select and; rectify the lower frequencies in the auxiliary sig-- nals applied to its' input circuit, ax relay having Va predetermined time delay in operationzand'means to-apply: the rectified signal output of said' second' varistor bridge through said. relay to` said one varistor bridge circuitv in. such manner as to: change the shunting impedance thereof on= said carrier channel to-a low value thereby inserting a,

high loss in said channel, said. relay being operatively responsive with a predetermined amount of delay' to the applied rectiedsignalsftov connect a direct shorten theinput. of saidlcliannel.

so as to'substantia'lly preventtransmissionof the'.

auxiliary signals and high. frequency' transients applied thereto.

81. In: combinationy a telephonechannel'.` a two-- wirei for feeding' Voice: frequency telephone signals to said channel', means for occasionally impressing auxiliaryr signa-lsf including voice and lower frequencies; on. saidl linel for.- ringing orv other switching purposes, a tuned relay selectively' responsive*- to f one of. said' lower frequencies. im said. auxiliary signa-ls connected across said.' line infront 'uf1'said1 telephone: channel" and means: tor limit noise interference in. said telephone channel' dna to: the. transmitted.. auxiliary signals comprising afvari'or-losser bridgedfacross said'twowire line. beyond; the point of connection of said. relay thereto, and normally providing.' a: relatively low' loss therein to voice. frequency tele phone signals,. tuned rectifying` means' connected inseries withf saidI relay" across said' line;v for lse lectiveiyf rectifying a .portion of said. auxiliary signals of: said lower frequencies. and means responsive to the rectified signals.: .for biasing' said'. vario1osser,.so that it; provides a low impedance shunt across, said liniev such as' to provide ahigh loss therein; in front ofv said? telephoney channel during auxiliary signaling: intervals.

9; The combination of claim 8,. in whichV a. re'- tardatiorr coiLisr'connected between: the output'. of said: rectirying means and said vario-lossen. said retardation. coil providing a. high. impedance to` said: one lower frequency* during non-signaling: intervals.

1.0. 'Ih'e combination'4 ofl claim' 8;. in which the necessary. loss tosaid lower frequencies between the line-shunting circuits .including saidl rectifying means and said; vario-lossen. respectively, to. prevent the low.` frequency signals suppliedf to said: relay from being: affected by.I the. action ofi` saidi 'varioz-losseiris: obtained bytuning the elements in serie'srwith the shunting circuit includ-- ing said rectifying means tofsaid one lower frequency; andi thel elements. in series withy the shunting circuit including said vario-losser' tol some` higher frequency;

ARTHUR L. BONNER.

REFERENGE-S CITED.

The` following references are ofV record in the' le 'of this Ipartent? UNITEDV STATES' PATENTS Number' Name Date` 137333130 Miles et al. Oct. 29,1929" 2,004,759 Jester' June 11, 1935 '2`,O20;487` Wadsworth et al. Nov. 12, 1935 2,099,250 Adler Nov; 16, 1937 2;323;69`2" Waller July 6, 1943 2",`34 0;366` Cesaro' Feb. 1, 1944 2,355g642 Gose Aug. 15', 1944` 

